1. FIELD OF THE INVENTION.
The present invention relates to a process for the preparation of polyolefins, especially polyethylenes, having a multimodal molecular weight distribution, more particularly a bimodal molecular weight distribution.
2. DESCRIPTION OF RELATED ART.
Polyolefins such as polyethylene which have high molecular weight generally have improved mechanical properties over their lower molecular weight counterparts. However, high molecular weight polyolefins can be difficult to process and can be costly to produce. Polyolefins having a bimodal molecular weight distribution are desirable because they can combine the advantageous mechanical properties of high molecular weight fraction with the improved processing properties of the low molecular weight fraction.
For many HDPE applications, polyethylene with enhanced toughness, strength and environmental stress craking resistance (ESCR) is important. These enhanced properties are more readily attainable with high molecular weight polyethylene. However, as the molecular weight of the polymer increases, the processibility of the resin decreases. By providing a polymer with a broad or bimodal MWD, the desired properties that are characteristic of high molecular weight resin are retained while processability, particularly extrudibility, is improved.
There are several methods for the production of bimodal or broad molecular weight distribution resins: melt blending, reactor in series configuration, or single reactor with dual site catalysts. Melt blending suffers from the disadvantages brought on by the requirement of complete homogenization and high cost. Use of a dual catalyst for the production of a bimodal resin in a single reactor is also known.
Chromium catalysts for use in polyolefin production tend to broaden the molecular weight distribution and can in some cases produce bimodal molecular weight distribution but usually the low molecular part of these resins contains a substantial amount of the comonomer. Whilst a broadened molecular weight distribution provides acceptable processing properties, a bimodal molecular weight distribution can provide excellent properties. In some cases it is even possible to regulate the amount of high and low molecular weight fraction and thereby regulate the mechanical properties. It is difficult to make bimodal polyethylene, for example, with a single catalyst because two separate sets of reaction conditions are needed. Frequently, two reactors are linked in series. However,this can give rise to processing problems. Where loop reactors are used, they are typically linked by a conduit which limits the throughput of material through the reactor system. This can result in an expensive product.
Ziegler Natta catalysts are known to be capable of producing bimodal polyethylene using two reactors in series. Typically, in a first reactor, a low molecular weight homopolymer is formed by reaction between hydrogen and ethylene in the presence of the Ziegler Natta catalyst. It is essential that excess hydrogen be used in this process and, as a result, it is necessary to remove all the hydrogen from the first reactor before the products are passed to the second reactor.
In the second reactor, a copolymer of ethylene and hexene is made so as to produce a high molecular weight polyethylene. Metallocene catalysts are also known in the production of polyolefins. For example, EP-A-0619325 describes a process for preparing polyolefins such as polyethylenes having a multimodal or at least bimodal molecular weight distribution. In this process, a catalyst system which includes at least two metallocenes is employed. The metallocenes used are, for example, a bis(cyclopentadienyl) zirconium dichloride and an ethylene-bis(indenyl) zirconium dichloride. By using the two different metallocene catalysts in the same reactor, a molecular weight distribution is obtained which is at least bimodal.
EP-A-0770629 relates to a process for producing an in situ polyethylene blend, the examples of which use a bis(cyclopentadienyl) zirconium ethyl chloride or indendyl zirconium tris(pivalate) catalyst. Truly bi- or multimodal polyolefin products are not obtained.
The present invention provides a process for the preparation of polyolefins having a bi- or multimodal molecular weight distribution which comprises:
EP-A-0605952 discloses an ethylene/alpha-olefin copolymer composition obtained from a multi-stage olefin polymerisation process involving a plurality of metallocene compounds.
The present invention aims to overcome the disadvantages of the prior art.
The present invention provides a process for the preparation of polyolefins having a bi- or multimodal molecular weight distribution which comprises: